(1) Field of the Invention
The present invention relates to a semiconductor RAM (Random Access Memory) device, more particularly to a static type RAM device which has a high read-out speed and a high reliability of read-out data.
(2) Description of the Prior Art
In recent years, the memory capacity and thus the integration, density of semiconductor memory devices has become very large. This is because it has become possible to make a great number of elements of very small size on one chip, due to recent technical developments in integrated circuitry. In a semiconductor memory device such as a static-type semiconductor RAM device having flip-flop type memory cells, the size of each of the memory cells has become very small so that the memory capacity has become very large.
However, in a conventional static type RAM device in which each of the memory cells drives a sense amplifier directly through bit lines and data bus lines, one disadvantage is that the read-out speed of the RAM device is decreased and the reliability of read-out data is decreased. This is due to the increase of the integration degree of the semiconductor memory device, where the chip size of each of the memory cells is small and, especially in a case of a static type memory cell, the size of the driver transistors included in each of the memory cells is small and hence the drive capacity of each of the memory cells decreases. Moreover, the stray capacitance of the bit lines and the data bus lines, which are driven by the memory cells and which form load circuits of the memory cells, does not decrease in accordance with the increase of the integration density of the RAM device. Therefore, the amplitude of a read-out signal from each of the memory cells becomes small, and the reliability of the read-out data is decreased. Thus, the time necessary to change the potential on each of the bit lines and the data bus lines becomes large, because it takes a long time to charge or discharge the stray capacitance of these lines by a memory cell having a small drive capacity. Therefore, the read-out time of the RAM device increases according to the increase in the integration density of the RAM device.